The present invention relates to an electrophotographic printer using an electrophotographic process.
FIG. 3 is a schematic side view of a main portion of a conventional electrophotographic printer as disclosed, for example, in Japanese Patent Unexamined Publication No. 61-95956, and FIG. 4 is a block diagram of the flow of a picture signal in such an electrophotographic printer.
In the drawings, reference numeral 1 designates a photosensitive drum, and 20 designates an LED head constituted by an LED array 2 and a lens array 3. The LED head 20 is attached to a frame (not shown) in such a manner that the light emitted from the LED array 2 is focused by the lens array 3 on the outer circumference of the photosensitive drum 1 and a group of the focuses forming a scanning line becomes parallel to the axis of the photosensitive drum 1. Thus, the surface of the photosensitive drum 1 is exposed at its position corresponding to the scanning line so that a latent image is written on the surface of the photosensitive drum 1 in accordance with a video signal supplied from a picture processing unit 11 which will be described later. Reference numeral 4 designates a charger for uniformly charging the surface of the photosensitive drum 1, 5 designates a developer for causing toner to adhere to the written latent image, 6 designates a transfer device for transferring the toner developed on the surface of the photosensitive drum 1 to recording paper 7, 8 designates a fixer for heating the transferred toner so as to fix the toner onto the recording paper 7, 9 designates a cleaning device for cleaning the surface of the photosensitive drum 1 after the toner has been transferred; 10 designates a discharger, 11 designates a host computer which is a source for generating a picture signal, and 12 designates a picture processing unit for conversion-processing the picture signal generated by the host computer 11. The picture processing unit 12 converts the picture signal by one page into picture data and stores the picture data in a RAM 13. The picture processing unit 12 takes out the stored data from the RAM converts it into a video signal and supplies the video signal to an LED driver 14. Reference numeral 15 designates a motor for rotating the photosensitive drum 1 at a predetermined speed.
In the thus configured electrophotographic printer, the picture data stored in the RAM 13 is taken out from the RAM 13 in response to a picture print command successively in accordance with the speed of rotation of the photosensitive drum 1, and the LED driver 14 causes the LED array 2 to emit light in form of a scanning line so that the surface of the photosensitive drum 1 is exposed to the light and the latent image is written on the surface of the photosensitive drum 1. The portion where the latent image has been written is rotated by the motor 15 in the direction shown by arrow A, the toner is caused to adhere to the portion of the latent image by the developer 5, and the toner is transferred by the transfer device 6 onto the recording paper 7. Next, the recording paper 7 is heated by the fixer 8 so that the toner is fixed onto the recording paper 7 obtain a completed picture.
In the conventional electrophotographic printer as described above, although, the speed at which the latent image is written by each scanning line is high, the speed of the entire printing process (the speed of rotation of the photosensitive drum 1) is restricted by physical conditions such as developing, transferring, fixing in order to make the quality of the printed picture good. Accordingly, in order to compensate for the speed difference, it has been necessary that the picture data for one page be stored in the RAM 13 and the stored data be consecutively read out.
Accordingly, there has been the problem that the RAM 13 is required. Since RAM 13 requires about 8 megabits per page of an A4 size paper in the case of 300 dpi (dot/inch) resolution, a large number of such memories make the apparatus expensive.